AP Syllabus focus:
‘El Niño and La Niña are linked to changes in Pacific Ocean surface temperatures.’
El Niño–Southern Oscillation (ENSO) describes recurring shifts in the tropical Pacific Ocean’s surface temperatures. These temperature changes alter ocean-atmosphere coupling, making ENSO a foundational concept for interpreting short-term climate variability.
Core idea: Pacific Ocean surface temperatures drive ENSO
ENSO is identified primarily by patterns in sea surface temperature (SST) across the equatorial Pacific, especially the central and eastern Pacific. Small average changes (often fractions of a degree to a few degrees Celsius) matter because the tropical ocean strongly influences the atmosphere above it.
Key terms used to describe ENSO temperature patterns
ENSO (El Niño–Southern Oscillation): A coupled ocean–atmosphere cycle in the tropical Pacific characterised by recurring warm (El Niño) and cool (La Niña) SST phases and associated shifts in atmospheric pressure and winds.
A central way to describe ENSO is by comparing observed SSTs to a long-term average for the same season.
Sea surface temperature (SST) anomaly: The difference between the measured ocean surface temperature and the expected long-term average (climatology) for that location and time of year.
“Normal” (neutral) tropical Pacific SST pattern
In neutral conditions, tropical Pacific SSTs are not uniform; they form a west-to-east gradient that supports a stable ocean-atmosphere circulation.
Three schematic cross-sections compare neutral (normal), El Niño, and La Niña states across the equatorial Pacific. The diagrams highlight how the warm surface “pool” shifts eastward during El Niño and retreats westward during La Niña, alongside changes in the thermocline slope that modulate eastern-Pacific cooling and warming. Source
Typical SST distribution (neutral)
Western equatorial Pacific: warmer SSTs (large pool of warm surface water).
Central/eastern equatorial Pacific: cooler SSTs because cooler subsurface water is brought upward more effectively.
This gradient helps maintain a consistent pattern of ocean mixing and surface currents along the equator.
Why SSTs differ from west to east
Surface winds push warm surface water westward, allowing warmer water to “pile up” in the western Pacific.
Near the equator, the ocean is sensitive to wind-driven changes because surface waters are easily displaced and replaced by subsurface waters.
Cooler surface conditions in the east are reinforced when deeper, cooler water influences the surface more strongly.
El Niño: warming of the central/eastern equatorial Pacific
El Niño occurs when SSTs in the central and/or eastern equatorial Pacific become warmer than average for an extended period. For AP Environmental Science, focus on the SST change itself and its immediate ocean-side meaning.
What “warm phase” means in SST terms
Positive SST anomalies develop in the central/eastern Pacific.
A global map of sea surface temperature anomalies illustrates the signature El Niño warming band along the equatorial Pacific, especially extending toward the eastern Pacific near South America. The color scale visualizes anomalies (departure from average), making it easier to interpret what “positive SST anomalies” look like in observational data. Source
The usual west-to-east SST gradient weakens (the east becomes less cool relative to the west).
Warmer-than-average surface waters can spread eastward along the equator, changing where the warmest waters are located.
Ocean conditions commonly associated with El Niño SSTs
The boundary between warm surface water and cooler deep water becomes less effective at keeping the east cool.
Reduced delivery of cool subsurface water to the surface supports sustained warming at the surface.
A NOAA Climate Prediction Center schematic summarizes typical El Niño (Dec–Feb) ocean–atmosphere structure. It emphasizes warmer-than-average surface water in the central/eastern equatorial Pacific and a deeper eastern-Pacific thermocline, which suppresses upwelling and helps maintain positive SST anomalies. Source
La Niña: cooling of the central/eastern equatorial Pacific
La Niña is the opposite SST phase: the central and/or eastern equatorial Pacific becomes cooler than average for an extended period.
What “cool phase” means in SST terms
Negative SST anomalies occur in the central/eastern Pacific.
The west-to-east SST gradient strengthens (the east becomes even cooler relative to the west).
Cooler surface waters persist across large areas, indicating enhanced influence of cooler subsurface conditions at the surface.
Ocean conditions commonly associated with La Niña SSTs
Stronger maintenance of cool surface waters in the eastern Pacific.
A more pronounced contrast between the warm western Pacific and cool eastern Pacific at the surface.
How scientists track ENSO using SSTs
ENSO phases are monitored by observing SST patterns over time rather than relying on a single day’s measurement.
Common monitoring approach (conceptual)
Track SSTs across defined equatorial Pacific regions (often “index regions”).
Compare observations to the seasonal expected average to compute SST anomalies.
Look for persistence (multi-month) and spatial extent (large continuous areas) of warming or cooling.
Why persistence matters for classification
Short-lived warming/cooling can occur from weather and storms.
ENSO classification depends on sustained, basin-scale SST anomaly patterns that reflect a coupled ocean state rather than brief surface fluctuations.
FAQ
Satellites estimate skin temperature using infrared/microwave sensors, while in-water networks (e.g., moored buoys and floats) measure bulk SST.
Combining sources improves accuracy when clouds, rainfall, or surface layering affect readings.
Many agencies use a sustained anomaly near $0.5^\circ\text{C}$ in a central Pacific index region for several overlapping seasons.
Exact thresholds and averaging windows differ by organisation, which can shift onset/ending dates.
Event “flavours” depend on where the strongest positive SST anomalies concentrate.
Differences relate to ocean wave dynamics, background currents, and how winds interact with pre-existing temperature patterns along the equator.
A thin surface layer can warm/cool rapidly due to changes in mixing, sunlight absorption, and evaporation.
If mixing weakens, surface warming can intensify even without large deep-ocean temperature changes.
Forecast skill drops due to:
springtime shifts in Pacific ocean-atmosphere coupling (“predictability barrier”)
sensitivity to short-lived wind bursts
model differences in representing upper-ocean mixing and cloud feedbacks
Practice Questions
State the key ocean variable used to identify El Niño and La Niña, and describe how it differs between the two phases. (2 marks)
Identifies sea surface temperature (SST) or SST anomaly as the key variable (1)
Correctly states El Niño = warmer-than-average (positive) SST anomalies and La Niña = cooler-than-average (negative) SST anomalies in the central/eastern equatorial Pacific (1)
Describe the typical (“neutral”) SST pattern across the equatorial Pacific and explain how that pattern changes during El Niño and La Niña. (5 marks)
Neutral: western equatorial Pacific is warmer than the eastern equatorial Pacific (1)
Neutral: describes a west-to-east SST gradient across the tropical Pacific (1)
El Niño: weakens the gradient by warming the central/eastern equatorial Pacific (1)
La Niña: strengthens the gradient by cooling the central/eastern equatorial Pacific (1)
Uses correct anomaly language (positive for El Niño and/or negative for La Niña) or clearly links phases to “warmer/cooler than average” over multiple months (1)
